Will not boot into safe mode

Hello,

My MacBook pro 2012 crashed last night. Could not force quite so closed it down. On restart it would get halfway then stop. I have japped p-ram (I know it is my called this anymore) but then loads halfway and the turns off. I booted into disk repair and it found a couple of problems so repaired successfully. On restart it still gets halfway then turns off.

From reading the apple support it seems other people have the same problem. All mention booting into safe mode. For the life of me I cannot get my machine to boot into safe mode!

I do exactly like the instructions say but it always just goes to the page where I choose whether to log in as me or a guest.

What am I doing wrong?!

I am on a MacBook pro running Yosemite.

Many thanks for any help.

Chris.